In recent years, progress of mass information recording techniques has been advanced on research and development of high density optical recording techniques capable of storing information as much as possible in a unit area. The optical disc technique having been applied practically to products so far focuses a laser light on an object disc through a lens to read out and/or record data from/on the disc. To realize such high density recording of data, efforts have been made to reduce the size of the focused laser spot. The spot size is proportional to λ/NA if the light source wavelength is defined as λ and the numerical aperture of the objective lens is defined as NA. In other words, the amount of information to be stored on a disc has been increased by decreasing the light source wavelength and increasing the lens NA. If a set of a light source wavelength, an objective lens NA, and a capacity of data stored in a disc of 12 cm in diameter is represented as (wavelength, NA, and capacity), (780 nm, 0.5, and 650 MB) is assumed for CDs and (650 nm, 0.6, and 4.7 GB) is assumed for DVDs. A technique that uses a blue laser light source has proposed two types of such sets; (405 nm, 0.85, and 25 GB) and (405 nm, 0.65, and 20 GB). This recording capacity makes it possible to record high definition TV image data for about 2 hours.
However, any of the above described recording capacities is insufficient for professional systems and security systems used, for example, in broadcasting stations. In those broadcasting stations, it is required to record more than 100 GB on one disc. There are image data required to be stored for a long period, for example, from several tens of years to about 100 years. Such mass data is required to be stored on one disc as much as possible because of the limit of places for having those media in the custody. The required capacity of one disc is several hundreds of GB to more than 1 TB.
Any of the above described recording methods will be difficult to cope with recording of such mass data because of the following problems. At first, realizing a short wavelength for light sources is difficult, because development of a semiconductor laser diode usable as such a light source is very difficult and even when such a semiconductor laser diode is developed successfully, the light source is of an ultraviolet light. Thus the disc substrate and the protective film come to absorb the light, thereby it is considered to be difficult to secure a favorable recording/reproducing quality. A research of increasing the objective lens NA is in progress now. For example, the Japanese Journal of Applied Physics Vol. 42, pp. 1101 to 1104 reports such a technique when the NA is assumed to be 1.8. In such a case, because a light used for recording/reproducing data is not an ordinary propagating light, but a light localized at a lens, which is referred to as a near-field light, this system is required to have a mechanism for getting the lens so close to the surface of the object disc and moving the lens above the disc while the distance between the lens and the disc surface is kept. The system is similar to a magnetic recording hard disk and the optical discs' merit, removability of discs, is sacrificed.
Under such circumstances, there has been proposed a method for improving the optical resolution effectively by providing a disc with a mechanism. Here, this method is referred to as a super-resolution technique.
The Japanese Journal of Applied Physics Vol. 32, pp. 5210 to 5213 reports such a super-resolution technique that uses a phase-change recording film. Usually, the phase-change recording film is used for the recording film of such rewritable discs as CD-ROM, DVD-RAM, DVD±RW, Blu-ray Discs. Here, this recording material is not used for a recording film, but used for a layer that improves the optical resolution effectively just like the read-out layer of the above described optical magnetic disc. The layer (film) is referred to as a super-resolution layer (film). This method deposits a phase-change recording film in a sputtering process and part of the recording film is melted upon reading out signals. If the reflectivity of the subject disc is higher enough at the melted portion, signals obtained from the melted portions become dominant over other signals. This means that phase-change film melted portions become effective readout light spots. Because the area of each melted portion is smaller than the optical spot, the readout optical spot is reduced substantially, thereby the optical resolution is improved.
The JP-A No. 2006-107588 proposes a method for obtaining such a super-resolution effect by advancing that method to form pits with a phase-change material and to melt individual pits upon reading out signals. According to this proposal, a phase-change etching method is used to form pits of the phase-change material. The phase-change etching is a technique for forming such pits by transforming a phase-change mark pattern to a pit pattern with good use of a difference of solubility between crystal portion and amorphous portion of the phase-change film with respect to an alkaline solution. According to this method, a substance that shows the super-resolution effect exists only in mark portions and the space portions are not required to absorb the light, so that the method can improve the optical transmittance of one layer and makes it possible to combine the super-resolution technique with a multi-layer technique. The Japanese Journal of Applied Physics Vol. 45, pp. 2593 to 2597 reports an example in which this method is used to realize a dual-layer super-resolution disc. This method is referred to as a pit type super-resolution technique and an example in which super-resolution films are deposited consecutively two-dimensionally as described above is referred to as a thin film super-resolution technique.
The Japanese Journal of Applied Physics Vol. 45, pp. 2593 to 2597 also reports an example that has realized a dual-layer super-resolution disc with another method. According to this method, a semiconductor is used as a super-resolution material. The band gap is in an ultraviolet light wavelength area at room temperature and in a visible light wavelength area at high temperatures. A thin film that absorbs a light is deposited around this super-resolution thin film. As a result, the temperature rises where the light spot intensity is high on the light absorbing thin film and the heat is transmitted to the super-resolution thin film, thereby the band gap of an area smaller than the light spot comes in a visible light area. Consequently, the readout light is reflected therefrom. In other words, readout signals are obtained only in areas smaller than the light spot, so that the super-resolution effect is obtained. In this example, ZnO is used as the super-resolution material.
A recording type super-resolution technique is also proposed. For example, the Japanese Journal of Applied Physics Vol. 43, pp. L8 to L10 reports a method for improving the recording density. According to the method, laser pulses are irradiated on a disc having both a platinum oxide film and a phase-change recording film using the same method as that for recording marks on an ordinary recordable optical disc, thereby recording marks, then reading out signals through a super-resolution reading-out process. This method irradiates a recording laser power on the object disc, then the platinum oxide film is expanded locally, thereby the thickness of the phase-change film is modulated according to each of the marks. Upon reading out signals, only the thin portions of the phase-change film are melted. Thus the super-resolution effect is obtained. Such way, this method realizes the write-once super-resolution optical disc.
On the other hand, the Japanese Journal of Applied Physics Vol. 37, pp. L516 to L518 reports a method for realizing a rewritable disc by adapting the method disclosed in the Japanese Journal of Applied Physics Vol. 32, pp. 5210 to 5213, which uses a phase-change film as a super-resolution film. This method reads out signals without erasing the phase-change marks recorded on the recording film while melting the super-resolution film upon reading out signals by using two types of phase-change films as a super-resolution film and a recording film respectively and adjusting the light absorptivity in each phase-change film according to the film thickness. This method selects a material so that the crystallization time of the phase-change material used for the recording film becomes slower than that of the super-resolution film. Consequently, recorded marks cannot be erased so easily upon reading out signals, thereby the required readout proof can be assured.
The JP-A No. 2001-273679 also discloses a method that provides the object medium with multiple layers and makes most use of the optical interference so as to maximize the reflectivity of the super-resolution area (aperture portion) in the light spot or minimizes the reflectivity of the non-super-resolution area (masking portion), thereby obtaining a higher super-resolution effect. This method is also aiming at increasing of the signal amplitude in the super-resolution reading-out process by paying attention only to the optical properties in the light spot.